purge: add a --confirm option
The options provide a prompt to the user before permanent deletion are made.
The prompt is currently not aware of directory deletion. I'll fix this in the
next changesets.
Differential Revision: https://phab.mercurial-scm.org/D9818
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)